PROJECT SUMMARY Hypoplastic left heart syndrome (HLHS), a severe congenital heart disease (CHD), is associated with high risk for neurodevelopmental disabilities. In fact, over 30% of HLHS survivors experience moderate to severe neurocognitive impairment. While brain abnormalities in HLHS patients are typically thought to be secondary to substrate delivery deficits from circulatory disturbance, our recent recovery of HLHS mutant mice with brain abnormalities point to a shared genetic etiology for the heart defects and the neurodevelopmental disabilities. This is supported by other studies showing CHD patients with de novo mutations in chromatin modifying genes are at increased risk for neurodevelopmental disorders that can include cognitive, motor, social and language impairments. We observed HLHS mutant mice to have brain abnormalities involving the cortex, hippocampus, and olfactory bulb, forebrain structures also frequently affected in HLHS patients. We showed HLHS and brain abnormalities in the Ohia mouse line have a digenic etiology, arising from mutations in two genes: Sin3a- associated protein 130 (Sap130), a chromatin modifying protein mediating transcriptional repression, and protocadherin a9 (Pcdha9), a protein involved in cell-cell adhesion. As chromatin modifying genes are already implicated in autism and also in neurodevelopmental impairment in CHD patients, insights into the role of Sap130 in the brain defects of the Ohia HLHS mice will have broad relevance for understanding the causes for poor neurodevelopmental outcomes in CHD and non-CHD patients. In this study, we will investigate the hypothesis that Sap130 deficiency perturbs brain development, causing brain dysmaturation with altered neural network connectivity and neurobehavioral deficits. In Aim 1, we will investigate the cellular and molecular mechanisms driving the brain dysmaturation, focusing on the forebrain. This will entail examining neurogenesis and cortical plate formation and conducting molecular profiling with RNAseq and ChIPseq analyses. In Aim 2, we will conduct multi-modal structural magnetic resonance imaging (MRI) that will include diffusion tensor imaging (DTI) to characterize the forebrain dysplasia and neural network connectivity changes contributing to the brain dysmaturation defects. To determine if behavioral defects may be elicited by the brain dysmaturation and neural network connectivity perturbations, in Aim 3 we will conduct a battery of rodent neurobehavioral assessments. The studies in Aims 2 and 3 will be carried out using a floxed Sap130 allele with forebrain targeted Cre mediated Sap130 deletion. This will allow mutant mice to survive to adulthood without heart defects. Such mice will be generated with or without the Pcdha9 mutation, allowing determination of the role of Pcdha9 in the brain abnormalities. Together these studies will yield new insights into the developmental etiology of the HLHS associated brain abnormalities and whether specific changes in neural architecture may drive the associated neurobehavioral/neurocognitive impairments. These findings may form the basis for future development of novel therapeutics that can substantively improve outcome for this vulnerable CHD population.